The invention relates to printing presses, and more particularly to a press having means for automated handling of sheet stock.
Transporting sheet stock in a sheet-fed press typically involves two steps: feeding, i.e., transporting sheets from an input station to a printing bed; and takeoff, i.e., removing printed sheets from the bed. One or both of these steps may be performed automatically.
In addressing the problem of handling sheet stock, a principal consideration is that apparatus for performing these functions must be capable of consistent precision and accuracy. A single misfeed may cause a series of subsequent misfed sheets and prevent any sheets from printing properly until the misfed sheets are removed.
Another concern is providing apparatus that is capable of high-speed operation. Mechanical efficiency tends to facilitate high-speed operation, and, accordingly, it is generally desirable that sheet-handling apparatus have no unnecessary moving parts, and no unnecessary movements.
Economy of manufacture and reliability are, of course, also essential to commercially viable equipment. These interests are also served by mechanical simplicity and efficiency.
One problem addressed by the present invention relates to automated closing of grippers on the edge of a sheet of stock. A mechanism for performing this function generally requires a relatively high degree of precision. If the grippers are closed too early, a variety of undesirable results may occur. The grippers may close before engaging the edge of the sheet of stock at all, or close on the sheet before their forward movement is complete, then push the sheet in the wrong direction for a short distance before beginning travel in the proper direction. Another possibility is that the grippers may obtain a partial grip on the stock, leading to dropping of the sheet during travel toward the printing bed, tearing of the stock, or improper registration of the sheet on the printing bed. Similar undesirable effects may result from late closing of the grippers.
Printing presses having automatic gripper assemblies for transporting sheets of stock generally employ unduly complex mechanisms to transport the grippers and open and close the grippers at desired points. These mechanisms may require excessive attention during set-up and adjustment to ensure proper operation. The present invention addresses this problem.
A second problem addressed by the invention relates to displacement of the printing bed to avoid interference between the bed and the grippers. The problem arises in presses which employ grippers that are disposed at such an elevation that their lower jaws are at least partially below the level of the printing bed in its printing position. The grippers move in a cycle, starting on one side of the printing bed and traveling rearward to pull a sheet over the bed, then opening to release the sheet and returning back over the bed to pick up the next sheet.
To avoid interference between the grippers and the printing bed, some presses in the past have employed means to lower the printing bed in rectilinear vertical travel from its upper, printing position to a lower position to permit the grippers to pass over the printing bed without interference. Typically, such mechanisms require that, after the grippers have opened to release the sheet and deposit it on the printing bed, the grippers move rearwardly, or else have their lower jaws retracted, to enable the sheet to drop vertically downward with the bed, without interference between the edge of the sheet and the lower jaws of the gripper during the downward travel of the sheet. After the sheet has dropped, the grippers may then travel forward, returning over the printing bed, to pick up the next sheet.
The output rate of presses operating as described above is limited by the complexity of the motion of the grippers. There remains a need for more efficient means to avoid interference between the grippers and the printing bed.
Accordingly, it is a general object of the invention to provide a press having improved sheet-handling mechanisms.
It is a further object to provide a press having improved means for controlling gripper opening and closing.
It is an additional object of the invention to provide a press having improved means to avoid interference between the grippers and the printing bed.
Further objects of the invention will become apparent from the description set forth below and the accompanying drawings.